What size space heater do I need?

How space heater sizing works

A common rule is 10 watts of heating per square foot for a room with 8 ft ceilings and average insulation — so 150 sq ft wants about 1,500 watts, which is the most a standard 120 V outlet can supply. Poorly insulated or high-ceilinged rooms need more; well-sealed rooms need less. If the calculator asks for more than 1,500 watts, you’ll want two heaters or a 240 V unit.